Enhance Triac Reliability Through Thermal Design

Sep 1, 2006 12:00 PM
By Nick Ham, Principal Applications Engineer, Bipolar Product Line, NXP Semiconductors, Hazel Grove,

Refrigerator Compressor Example

A triac is used in an electronic thermostat that controls the on-off switching of a refrigerator compressor. The triac gate is triggered from a microcontroller with 20-mA current sink capability. What maximum heatsink thermal resistance is allowed to keep the triac's junction temperature within its T_JMAX of 125°C? Steady-state motor current equals 1.4 A_RMS. Maximum inrush current equals 17 A_PK in the first half cycle. Mains supply equals 230 V_RMS. A surface-mounted triac is required for direct soldering to the controller pc board. Maximum ambient temperature is 40°C.

An 8-A Hi-Com triac is recommended to cope with the inductive load and startup current. A suitable triac is the BTA208S-600E, which uses the DPAK package. Its I_GATE of 10 mA is well matched to the drive capability of the microcontroller.

From the datasheet, V_O = 1.264 V and R_S = 0.0378 Ω.

Using Eq. 1, P = V_O × I_TRIACAVG + R_S × I_TRIACRMS² = 1.264 V × 1.26 A + 0.0378 Ω × (1.4 A)² = 1.67 W.

Using Eq. 6, T_JMAX = T_A + P × R_THJ-A.

T_JMAX = 125°C, T_A = 40°C and P = 1.67 W.

Rearranging the equation gives:

R_THJ-A = (T_J - T_A) / P = (125°C - 40°C) / 1.67 W = 51°C/W.

Using Eq. 7, R_THJ-A = R_THJ-MB + R_THMB-HS + R_THH-SA.

From the datasheet, R_THJ-MB = 2°C/W. We need to find R_THMB-A.

Rearranging the equation gives:

R_THMB-A = R_THJ-A - R_THJ-MB = 51°C/W - 2°C/W = 49°C/W.

This is effectively the heatsink thermal resistance, since the pc board is the heatsink in this case. As an approximate guide, this thermal resistance can be obtained with a copper pad area of 500 mm² (refer to the NXP application note “Surface Mounted Triacs and Thyristors,” document order number 9397 750 02622).

Please note that the actual thermal resistance will be reduced by other, nondissipating components in close proximity to the triac, while it will be increased by any components that dissipate power. It is essential to measure the prototype to discover the true thermal performance.

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